[TUHS] Early non-Unix filesystems?

Johnny Billquist bqt at update.uu.se
Fri Mar 25 09:06:35 AEST 2016

On 2016-03-24 23:50, Peter Jeremy wrote:
> On 2016-Mar-24 11:17:18 +0100, Johnny Billquist <bqt at update.uu.se> wrote:
>> It is the normal behavior of any instruction that interrupts are not
>> recognized until the next instruction fetch. This is how the microcode
>> works, and it is also pretty much the same in any processor today.
> ...
>> individual instructions. You still get a fetch between each instruction,
>> at which point, interrupts will be recognized.
> Some instructions inhibit the "check for interrupts at the end of this
> instruction" check.  I'm most familiar with the 8080 EI instruction,
> which enabled interrupts after the following instruction (so things like
> EI;HLT didn't have a window).  It seems the PDP-11 SPL behaves the same.

I don't think it should on the PDP-11, and the documentation do not 
mention any such thing.
There is a good reason the 8080 (and Z80, and others) have this 
property. The RETI instruction on these machines do not enable 
itnerrupts themselves, so just as you note, you need to both enable 
interrupts and return from interrupt atomically, or else you get into a 

The PDP-11 RETI instruction changes the processor priority as a part of 
the instruction. You do not use SPL (whatever) before a RETI.
Thus, it do not make sense that SPL on a PDP-11 would have this 
property. If if indeed do disable recognizing interrupts after an SPL, 
it sounds more like a bug. I guess I'll go and read the microcode so see 
if that mentions any of this, since I'm sortof into reading it anyway as 
I was trying to debug a problem on an 11/70 only a couple of months ago...

I'll try and remember to report back when I know something more.

>> memory access to get the actual content. The fun thing happens if you
>> set the indirect bit, and give your own address. This is then an
>> infinite memory reference. And the KA10 can not be broken out of that
>> lookup. The only solution is to pull the power plug.
> I can't think of any modern architectures that still support this sort
> of indefinite indirection but I know the ITT 3200 (custom CPU for
> controlling telephone exchanges) could do this.  In it's case, a normal
> front-panel reset would recover.

Yeah. The fact that the microcode didn't even recognize any signals from 
the front panel while in this loop was bad, they realized... :-)


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